The genes of the mitochondrion (kinetoplast) of some Kinetoplastida - e.g. Trypanosoma brucei, the parasite which causes African sleeping sickness - are "encoded" in a way that the pre mRNA requiries partially heavy editing (insertion and deletion of Uridines) by the editosome (a RNA/Protein complex) until it can be translated to the correct proteins. The information where to insert and remove Uridines is contained in small gRNA also part of the mitochondrial genome.
However this whole thing seems kind of unnecessary to me. Why did evolution not get rid of the surplus/missing Thymidines in those Genes? What's the use of this?